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Re: GSSAPI SMTPD Authentication and MS Active Directory

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  • Viktor Dukhovni
    ... What evidence do you have that the server is doing GSSAPI? It seems likely you re mistaken. Simply listing GSSAPI as a supported SASL AUTH mechanism is
    Message 1 of 8 , Apr 25, 2013
      On Thu, Apr 25, 2013 at 12:27:59PM -0700, Matthew Larsen wrote:

      > >
      > > If you want to use SASL/GSSAPI, the clients have to be able to get
      > > a TGT from the KDC.
      > >
      >
      > The reason I've been looking at configuring the SASL/GSSAPI
      > mechanism is that's what I see the current Exchange server doing.

      What evidence do you have that the server is "doing" GSSAPI? It
      seems likely you're mistaken. Simply listing GSSAPI as a supported
      SASL AUTH mechanism is not "doing" GSSAPI, the client would actually
      have to use GSSAPI. It is quite possible your client's IP address
      was whitelisted on the Exchange servers, or access was unrestricted, ...

      > I'm just puzzled as to how this works because the clients aren't
      > members of our AD domain, and I strongly doubt they have data for,
      > or access to, the DNS servers in the domain or a KDC. All they are
      > given is an SMTP server, username (DOMAIN\Username), and password.

      The clients may be doing NTLM or PLAIN or nothing at all. You need
      to figure out what's actually used. If TLS is not in use a simple
      packet capture plus wireshark or similar will show you exactly what
      the client and server are doing.

      > I'm not sure that my understanding of the security of the GSSAPI
      > method is accurate, or that the infrastructure is there in this case
      > to support doing this with Postfix?

      The Postfix SMTP client if compiled with Cyrus SASL support, and
      provided the Cyrus SASL gssapi plugin is installed will do GSSAPI.
      There is no GSSAPI-specific code in Postfix, all the logic is in
      Cyrus SASL. However, you need to specify a KRB5CCNAME in the
      client's environment that is readable by the "postfix" user and
      contains valid tickets at all times. To do this, run a cron-job
      periodically that uses a keytab file to populate the credential
      cache with freshly valid tickets.

      If the above is just a bunch of greek to you, you want to look for
      alternatives to GSSAPI.

      > I'll check out the LDAP authentication setup. Hopefully as I gain
      > a better understanding of other possible pieces of this
      > configuration the whole thing will start to gel together for me.

      If you replace the Exchange servers with Postfix, you can support
      any of the following authorization methods:

      - Allow any client to send anywhere (internal open relay).
      - Whitelist the particular sending IPs.
      - Allow the clients to send via authorized TLS client certs.
      - Allow the clients to send via any mutually supported SASL
      mechanism, including PLAIN and/or GSSAPI.

      For server-side GSSAPI support the server will need a keytab file
      containing shared keys with the appropriate realm's KDCs.

      --
      Viktor.
    • Matthew Larsen
      ... My apologies. I am mistaken about how this is happening. Sometimes it s a challenge to get accurate information from a different division that takes care
      Message 2 of 8 , Apr 25, 2013
        On 4/25/2013 1:02 PM, Viktor Dukhovni wrote:
        > What evidence do you have that the server is "doing" GSSAPI? It
        > seems likely you're mistaken. Simply listing GSSAPI as a supported
        > SASL AUTH mechanism is not "doing" GSSAPI, the client would actually
        > have to use GSSAPI. It is quite possible your client's IP address
        > was whitelisted on the Exchange servers, or access was unrestricted, ...

        My apologies. I am mistaken about how this is happening. Sometimes it's
        a challenge to get accurate information from a different division that
        takes care of this client system.

        The computers running the SMTP client software are members of a child
        domain in our AD forest, there's a VPN between those computers and a
        different segment of our network housing the child domain AD
        infrastructure, but for some reason (probably bandwidth and latency) the
        SMTP client is connecting over the public Internet connection at the
        client sites rather than the VPN. I think that mostly explains how the
        infrastructure is there to use Kerberos for authentication.

        Here's what I see it doing with wireshark on the server.

        A screen shot of some of what I see:
        http://img94.imageshack.us/img94/2579/gssapismtpauth.png

        The gist of it is

        S: 220 mail.exch01.com ...
        C: EHLO NETBIOSName
        S: 250-mail.exch01.com Hello [ip.addr.of.client] | 250- ... several
        items including AUTH GSSAPI NTLM LOGIN among others ....
        C: AUTH gssapi ...long string...
        S: 334 ...long string...
        C: ...long string...
        S: 235 2.7.0 Authentication successful.
        C: MAIL FROM:<sending@...>
        S: 250 2.1.0 sending@... ... Sender OK
        C: RCPT TO:<somebody@...>
        S: 250 2.1.5 somebody@...
        C: DATA
        S: 354 Start mail input; end with <CRLF>.<CRLF>
        ... blah blah blah ...

        > The clients may be doing NTLM or PLAIN or nothing at all. You need
        > to figure out what's actually used. If TLS is not in use a simple
        > packet capture plus wireshark or similar will show you exactly what
        > the client and server are doing.

        In addition to what I see in Wireshark, the event log shows it's using
        GSSAPI when I turn on the MSTransport authentication logging level to debug.

        Event Type: Information
        Event Source: MSExchangeTransport
        Event Category: Authentication
        Event ID: 1708
        Date: 4/25/2013
        Time: 11:17:49 AM
        User: N/A
        Computer: EXCH01
        Description:
        SMTP Authentication was performed successfully with client "A510E". The
        authentication method was "GSSAPI" and the username was "MYDOMAIN\AAA".


        >> I'm not sure that my understanding of the security of the GSSAPI
        >> method is accurate, or that the infrastructure is there in this case
        >> to support doing this with Postfix?
        > The Postfix SMTP client if compiled with Cyrus SASL support, and
        > provided the Cyrus SASL gssapi plugin is installed will do GSSAPI.
        > There is no GSSAPI-specific code in Postfix, all the logic is in
        > Cyrus SASL. However, you need to specify a KRB5CCNAME in the
        > client's environment that is readable by the "postfix" user and
        > contains valid tickets at all times. To do this, run a cron-job
        > periodically that uses a keytab file to populate the credential
        > cache with freshly valid tickets.
        >
        > If the above is just a bunch of greek to you, you want to look for
        > alternatives to GSSAPI.

        It's not entirely greek, but I'm trying to learn more greek. However, I
        don't believe that I need the Postifix client to do any authentication
        other than anonymous. It would be relaying messages from authenticated
        clients to Internet recipients via MX records. I'm only trying to
        configure the stmpd portion of Postfix for secure authentication.

        > If you replace the Exchange servers with Postfix, you can support
        > any of the following authorization methods:
        >
        > - Allow any client to send anywhere (internal open relay).
        > - Whitelist the particular sending IPs.
        > - Allow the clients to send via authorized TLS client certs.
        > - Allow the clients to send via any mutually supported SASL
        > mechanism, including PLAIN and/or GSSAPI.
        >
        > For server-side GSSAPI support the server will need a keytab file
        > containing shared keys with the appropriate realm's KDCs.

        The fourth option listed is what I'm trying to accomplish with GSSAPI,
        but have been finding challenging to get working. I'll go back over my
        configuration a time or two and try and find something specific that
        will point to where it's not working.
      • Viktor Dukhovni
        ... So GSSAPI it is and the clients already have GSS credentials. ... You ll need to use the Microsoft command-line tools for to create SPN s (service
        Message 3 of 8 , Apr 25, 2013
          On Thu, Apr 25, 2013 at 02:39:28PM -0700, Matthew Larsen wrote:

          > The gist of it is
          >
          > S: 220 mail.exch01.com ...
          > C: EHLO NETBIOSName
          > S: 250-mail.exch01.com Hello [ip.addr.of.client] | 250- ... several
          > items including AUTH GSSAPI NTLM LOGIN among others ....
          > C: AUTH gssapi ...long string...
          > S: 334 ...long string...
          > C: ...long string...
          > S: 235 2.7.0 Authentication successful.

          So GSSAPI it is and the clients already have GSS credentials.

          > >If the above is just a bunch of greek to you, you want to look for
          > >alternatives to GSSAPI.
          >
          > It's not entirely greek, but I'm trying to learn more greek.
          > However, I don't believe that I need the Postifix client to do any
          > authentication other than anonymous. It would be relaying messages
          > from authenticated clients to Internet recipients via MX records.
          > I'm only trying to configure the stmpd portion of Postfix for secure
          > authentication.
          >
          > >If you replace the Exchange servers with Postfix, you can support
          > >any of the following authorization methods:
          > >
          > > - Allow any client to send anywhere (internal open relay).
          > > - Whitelist the particular sending IPs.
          > > - Allow the clients to send via authorized TLS client certs.
          > > - Allow the clients to send via any mutually supported SASL
          > > mechanism, including PLAIN and/or GSSAPI.
          > >
          > >For server-side GSSAPI support the server will need a keytab file
          > >containing shared keys with the appropriate realm's KDCs.
          >
          > The fourth option listed is what I'm trying to accomplish with
          > GSSAPI, but have been finding challenging to get working. I'll go
          > back over my configuration a time or two and try and find something
          > specific that will point to where it's not working.

          You'll need to use the Microsoft command-line tools for to create
          "SPN"s (service principals) for smtp/<hostname> for each new host
          on which you plan to install Postfix. Then another tool to extract
          a keytab file for each SPN. The keytab file will need to installed
          mode 0600 owned by "postfix".

          The Postfix SMTP server will need:

          import_environment = ... KRB5_KTNAME=FILE:/path/of/keytab/file

          where "..." includes all the default values of import_environment. It
          is also possible to delegate all the work of doing GSSAPI auth to dovecot,
          in which case the dovecot keytab will need to contain keys for both
          imap and smtp (or perhaps just smtp if dovecot is not used for imap),
          or choose gssapi as the mechanism in smtpd.conf for Cyrus SASL.

          The clients will need to be reconfigured to connect to a new set of
          server hosts.

          --
          Viktor.
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